As radiation therapy for cancer, there is particle therapy in which a tumor volume of a patient is irradiated with a charged particle beam such as a proton or a heavy ion so as to be treated. A particle therapy system used for such treatment includes a charged particle beam generator, a beam transport system, and an irradiation system. The charged particle beam generator includes a synchrotron or a cyclotron which accelerates a charged particle beam which orbits along a closed orbit to a target energy. An extraction method of a charged particle beam accelerated by a synchrotron has been heavily developed hitherto. PTL 1 and NPL 1 disclose an operation method in which a radiofrequency voltage with a single frequency matching a betatron oscillation frequency of a charged particle beam orbiting around a stability limit is fed in a superimposed manner along with an extraction radiofrequency voltage of the related art so that a particle around the stability limit is preferentially extracted during beam extraction control, and thus the density of charged particle beams around the stability limit is reduced during stoppage of beams in order to prevent the charged particle beams from leaking.